The present invention generally relates to a method and apparatus for chemical mechanical polishing and more particularly, relates to a method and apparatus for preventing metal corrosion on an electronic substrate during a chemical mechanical polishing process when the process is temporarily halted due to tool alarm by removing slurry solution from the substrate surface.
Method and apparatus for polishing thin, flat semi-conductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head rotates and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a walfer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer.
The mechanism for chemical mechanical polishing of metal is different and more complex than the polishing of silicon oxide. It is generally believed that during the CMP of metal, metal form an oxide layer on the surface which is subsequently removed by the polishing pad by a mechanism similar to that for oxide polishing. For instance, a mechanism that involves hydroxylation, bond formation with slurry and then, bond breaking from wafer. After the metal oxide layer is removed from the metal surface, metal is etched by the chemicals in the slurry solution, while simultaneously the exposed metal forms a new passivation layer through oxidation by the slurry solution. In practice, it is believed that three processes, i.e. the removal of metal oxide, the metal etching and the metal passivation occur simultaneously. A polishing slurry solution for use in metal CMP therefore contains three major components of fine slurry particles, a corrosion or etchant agent and an oxidant. The eventual planarization of the metal surface is achieved by the rigidity and planarity of the polishing pad similar to a process of oxide polishing.
When the metal being polished in the CMP process is copper, the polishing process becomes more complicated due to the characteristics of copper. Since copper is frequently used in multi-level interconnect structures in semiconductor devices, i.e. in damascene or dual damascene structures, a CMP step for forming copper interconnects in the damascene structures with satisfactory polishing uniformity becomes an important link in the entire fabrication process. The copper CMP process produces a fresh copper surface which is susceptible to corrosion when contacted by a variety of chemicals, including those contained in a slurry solution.
The corrosion of a metal layer on a wafer surface is more severe when the metal layer is formed of copper. Since in other metals such as tungsten and aluminum, a thin layer of metal oxide will be formed to stop further oxidation of fresh metal under the oxide layer. The formation of copper oxide does not stop the further oxidation of fresh copper under copper oxide. As a result, when the fresh copper surface is in contact with a CMP slurry solution for any significant length of time, i.e., more than three minutes, deep pitted areas of, copper oxide are formed. The depth of the copper oxide areas formed is so large such that any repair attempted on the wafer surface would not be effective to save the wafer from being scrapped. It therefore presents a serious problem in chemical mechanical polishing a copper layer on a wafer surface when the wafer surface may be subjected to a stationary contact with a slurry solution for any extended length of time, such as in a tool alarm situation wherein the polishing apparatus is shut down temporarily for correcting mechanical or other problems.
It is therefore an object of the present invention to provide a method for chemical mechanical polishing copper that does not have the drawbacks or shortcomings of the conventional copper CMP process.
It is another object of the present invention to provide a method for chemical mechanical polishing copper on a semiconductor device that does not have copper corrosion problem.
It is a further object of the present invention to provide a method for chemical mechanical polishing copper on a semiconductor device that does not have copper corrosion problem even when the process is temporarily halted due to a tool alarm situation.
It is another further object of the present invention to provide a method for prevent meal corrosion on a wafer during a chemical mechanical polish process by spraying a cleaning solvent toward the wafer surface removing slurry solution when the polishing process is stopped.
It is still another object of the present invention to provide a method for preventing copper corrosion on a wafer surface during a chemical mechanical polishing process when the process is temporarily halted due to tool alarm by spraying deionized water toward the wafer surface and removing slurry solution such that copper corrosion can be prevented
It is yet another object of the present invention to provide a method for preventing copper corrosion on a wafer surface during a chemical mechanical polishing process by installing spray nozzles in the polishing apparatus and spraying a cleaning solvent onto the wafer surface for removing slurry solution during a temporary shut-down.
It is still another further object of the present invention to provide a polishing apparatus for polishing an electronic substrate on a polishing pad with a slurry solution to dispensed thereinbetween which includes at least one spray nozzle for dispensing a cleaning solvent toward the electronic substrate for removing slurry solution from the substrate during a temporary shut-down of the polishing apparatus.
It is yet another further object of the present invention to provide a chemical mechanical polishing apparatus for polishing a silicon wafer that has a copper layer deposited thereon which includes a cleaning apparatus for dispensing a cleaning solvent and removing slurry solution from the wafer surface during a temporary shut-down of the polishing apparatus to prevent corrosion of the copper layer.
In accordance with the present invention, a method for preventing metal corrosion on a wafer during a chemical mechanical polishing process when the process is temporarily halted due to tool alarm and an apparatus for carrying out such method are provided.
In a preferred embodiment, a method for preventing metal corrosion on a wafer during a chemical mechanical polishing process when the process is temporarily halted due to tool alarm can be carried out by the operating steps of providing a wafer that has a metal layer on a top surface to be polished by a polishing pad and by a slurry solution, the metal layer is susceptible to corrosion when exposed to the slurry solution in a stationary manner, mounting the wafer in a CMP apparatus with the metal layer on the wafer exposed, polishing the metal layer on the wafer by rotating against a polishing pad with a slurry solution dispensed thereinbetween, stopping the polishing process by the occurrence of a condition, and spraying a cleaning solvent toward the wafer surface and removing substantially all slurry solution from the wafer surface to prevent corrosion of the metal layer by the slurry solution.
The method for preventing metal corrosion on a wafer during a CMP process when the process is temporarily halted may further include the step of spraying a cleaning solvent of deionized water toward the wafer surface, or the step of spraying deionized water at a pressure between about 1 PSI and about 20 PSI toward the wafer surface. The method may further include the step of providing a plurality of spray nozzles for spraying the cleaning solvent, or the step of providing a plurality of spray nozzles mounted on bendable, shapable conduit for spraying the cleaning solvent. The method may further include the step of disengaging the wafer from the polishing pad such that the top surface of the wafer is exposed after the polishing process is stopped. The method may further include the step of providing the wafer with a copper layer on a top surface of the wafer.
The present invention is further directed to a polishing apparatus for polishing an electronic substrate on a polishing pad with a slurry solution dispensed thereinbetween, a substrate holder for holding an electronic substrate therein exposing an active surface of the substrate to be polished, the active surface of the substrate is at least partially covered by a metal layer, a polishing pad for rotating at a rotational speed of at least 10 RPM while engaging the active surface of the electronic substrate, a slurry solution for enhancing polishing of the active surface of the electronic substrate by the polishing pad, and at least one spray nozzle for dispensing a cleaning solvent towards the active surface of the electronic substrate when the substrate holder is disengaged and separated from the polishing pad during a temporary shut-down of the polishing apparatus for removing slurry solution from the active surface of the electronic substrate and preventing etching of the metal layer by the slurry solution.
The polishing apparatus of the present invention may be a chemical mechanical polishing machine. The at least one spray nozzle may further include a bendable, shapable conduit for mounting the spray nozzle and for transporting the cleaning solvent. The electronic substrate may be a silicon wafer, while the conductive layer on the electronic substrate may be a copper layer. The cleaning solvent dispensed toward the active surface of the electronic substrate may be deionized water. The active surface of the electronic substrate may further include a single-damascene interconnect structure formed of copper, or a dual-damascene interconnect structure formed of copper. The at least one spray nozzle may be mounted on a platform onto which the polishing pad is mounted at a position juxtaposed to the polishing pad.